MPV is one of my favorite open source projects. It's a simple-looking video player with an enormous amount of features while also being highly extensible through the lua programming language.
MPV-splice is a script that allows you to use MPV as a video editor. You set the start and end timings for fragments, and then the script cuts the video and concatenates the fragments.
I made a fork of it to extend it's abilities.
Another useful script I forked is 8mb, which compresses your video to 8mb, changing it's FPS and resolution to appropriate values.